1. Field of the invention
The present invention concerns a method of sequencing digital signal processing in the reduced rate communication mode of a digital cellular radio system using time-division multiple access, for example the panEuropean cellular radiotelephone system.
2. Description of the prior art
The digital signal processing will now be explained with reference to a terminal in order to clarify the disclosure, although this must not be seen as limiting the invention in any way. The terminal exchanges information by means of radio signals with a base station which is an access point to the cable communication network. It therefore comprises a radio circuit which converts the receive radio signal into a baseband digital signal and carries out the converse operation to produce the send signal. It also comprises a signal processing circuit which carries out various operations on the baseband receive signal to produce a digital speech signal to be routed to a converter circuit and which carries out the converse operations on the send signal. The converter circuit carries out the digital-to-analog and analog-to-digital conversions constituting the interface between the microphone and the loudspeaker of the terminal, on the one hand, and the signal processing circuit, on the other hand. The terminal also comprises a control circuit which directs the operation of the terminal, including the radio circuit and the signal processing circuit. Hereinafter the expression digital signal processing is to be understood as referring to all of the operations carried out by the signal processing circuit.
The expression communication mode is to be understood as meaning the operating mode after call set-up is completed, whether communication is at the full rate or at a reduced rate (these concepts will be explained later).
In other words, the following explanation presupposes that the call set-up procedures have been accomplished already (they do not form any part of the present invention).
The radiotelephone systems of interest are so-called time-division multiple access systems. These use repetitive frames subdivided into time slots. Full rate communication between a base station and a terminal takes place over a communication channel comprising two separate time slots in each frame, one for each transmission direction, these time slots always occupying the same position within the frame.
One specific system using this method is the panEuropean digital cellular radiotelephone system defined by the Groupe Special Mobile (GSM) of the European Telecommunications Standards Institute (ETSI).
A reduced rate communication mode is provided in which a plurality of terminals (two terminals, for example) share the same communication channel. The two terminals use the same time slots to send and receive but in different frames. One possible solution is to assign the first and second frames to the first terminal, the third and fourth frames to the second terminal, the fifth and sixth frames to the first terminal, and so on.
The invention will be explained with reference to this particular system, so that it is possible to refer to a known infrastructure, in particular the recommendations published by the GSM.
It follows that:
the baseband digital signal is a digital signal modulated by Gaussian Minimum Shift Keying (GMSK) at 270.3 Kbit/s, PA1 the speech digital signal comprises samples each of 13 bits at 8 kHz, PA1 communications with the control circuit essentially concern process commands, signalling and synchronization information.
The GSM system uses two overlaid time structures, namely multiframes comprising either 26 frames or 51 frames and called by convention 26 multiframes and 51 multiframes, respectively. The 51 multiframe is used in the call set-up procedure. The 26 multiframe is used to route the calls.
In communication mode the terminal therefore uses the 26 multiframe, call set-up and the change from the 51 multiframe to the 26 multiframe being explained in GSM recommendations of series 04 and 05. The various functions to be implemented in this mode can be divided into send functions and receive functions:
- send functions:
+ send speech processing: production by encoding of a speech block of approximately 130 bits from 160 speech samples each of 13 bits, which represents a half rate call to use the GSM terminology (the processing for the full rate is specified GSM recommendations 06.10, 06.12, 06.31 and 06.32).
+ channel coding: production of a coded block of 228 bits from a speech block (the coding for the full rate is specified in GSM recommendation 05.03).
+ interleaving: formation of an interleaved block of 232 bits from sub-blocks of two coded blocks (full rate interleaving is specified in GSM recommendation 05.03).
+ encipherment: producing packets of 116 bits from the interleaved blocks, each packet comprising two sub-blocks (GSM recommendation 03.20).
+ packet processing: production of a burst in the time slot format by adding control bits to a packet (GSM recommendations 05.01 and 05.02).
+ GMSK modulation: digital modulation of the bits forming the bursts (GSM recommendation 05.04).
- receive functions:
+ GMSK demodulation: this function is the counterpart of GMSK modulation and recovers the packets.
+ decipherment.
+ de-interleaving.
+ channel decoding.
+ receive speech processing: recovery of 160 samples each of 13 bits at 8 kHz from a speech block of approximately 130 bits.
The solution usually adopted is to execute these various functions in parallel in a so-called multitasking sequence which means using dedicated units for each function or group of functions, these units being able to operate simultaneously. The functions are clearly delimited and appear as such in the GSM recommendations. This solution is further justified by the complexity of the functions which leads naturally to considering them individually and also considering separately the means of generating them.
However, multitasking is characterized by the following features. Firstly, it imposes a complex structure of interrupt management at the level of the control circuit in order to coordinate the implementation of the various functions in the various units. Secondly, it requires the use of large buffers to store intermediate results generated by the various functions. Thirdly, it increase the complexity of synchronization of signal processing relative to that of the GSM system. One immediate consequence of these characteristics is the need to provide dedicated modules to meet the requirements of multitasking, which increases the surface area of silicon used. This is not desirable with regard to the cost or size of the circuit, factors of particular importance in the case of portable terminals.
Accordingly, one object of the invention is a method of sequencing digital signal processing which is of the single-tasking or serial type.